Enhanced sediment purification techniques for the <10 µm fraction of diatom silica and a comparison of contamination assessment before analysing oxygen isotopes

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Chapligin, B. , Meyer, H. , Hoff, U. , Diekmann, B. , Eulenburg, A. and Hubberten, H. W. (2009): Enhanced sediment purification techniques for the <10 µm fraction of diatom silica and a comparison of contamination assessment before analysing oxygen isotopes , 11th International Paleolimnology Symposium, Guadalajara, Mexico, December 15-18 .
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The analysis of oxygen isotopes of biogenic silica as a paleolimnological-proxy is of growing interest. In lacustrine environments the main particle and diatom size is often to be found in the <10 &#956;m fraction. Among other contaminants mineral particles cause a serious and undesirable shift in &#948;18O values and should be removed prior to the analysis.This is particularly difficult in the fraction containing clay particles. As so far thefraction <5 &#956;m was simply removed in all published methods, an updated preparation protocol for this fraction based on known methods had to be developed. After each purification step the material was analysed for the isotopic composition and the degree of contamination.The contamination can be determined either by optical methods under light microscope, or scanning-electron microscope (SEM) or by analysing the chemical composition (SiO2 content [%]) with x-ray fluorescence (XRF) analysis. Two more possibilities for assessing the element composition were tested additionally as they do not essentially require much material: Inductively coupled plasma optical emission spectroscopy (ICPOES)and Energy-dispersive-X-ray spectroscopy (EDX) under the SEM. The differentmethods were compared to find the fastest and most precise determination. The test material originates from short cores of Lake Elgygytgyn, NE Russia. In addition, internal biogenic standards were used to verify the accuracy of the chemical analyses.The EDX analysis provided the fastest results but with a higher mean standard deviation (1&#963; < 0.5 % for SiO2 > 95 %, n=6). The detection limit for this analysis is comparably high (app. 0.1 %) but still suitable for this purpose to verify samples having an SiO2 content greater than app. 97 %. The preparation process for the ICP-OES is timeconsuming but the method proved to give more accurate results (1&#963; < 0.2 % for SiO2 > 95 %, n=4). No significant difference could be detected when comparing the analysis of 50 mg to 10 mg of the same material using ICP-OES. The optical methods do not provide exact, quantified results for the lesser than 10 &#956;m fraction. The EDX can be combined with the optical SEM for detail shots and is the most recommended analysis after this comparison: It is the quickest method and needs less than 0.5 mg weighted sample. Still, its precision is good enough for the purpose of assessing the degree of contamination to implement potential correction factors. The updated protocol for removing contaminants in the <10 &#956;m fraction is able to gain a SiO2 content greater than 97% (analysis by EDX). When looking at the development of the isotopic composition an asymptotic approximation towards &#948;18O values not being influenced by contaminants is expected. First results will be presented at the Paleolimnology Symposium.

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